752  Dr.  E.  S.  Johonnott  on  the 
scope,  in  the  arrangement  shown  in  fig.  1,  it  is  oftentimes 
possible  to  distinguish  as  many  as  five  black  films.  These 
break,  one  into  another,  in  the  same  manner  in  which  the 
black  film  is  ordinarily  formed,  each  succeeding  film  being, 
apparently,  the  thinner.  The  first  three  formed  are,  how- 
ever, quite  evanescent  and  seldom  extend  over  an  elevation 
greater  than  two  or  three  millimetres.  The  fourth  and  fifth 
are  identical  with  those  which  have  been  termed  the  first  and 
second  black  films,  respectively.  Oftentimes  all  five  films  are 
in  the  field  of  view  at  once. 
Speculations. — The  apparent  continuity  of  the  surface- 
tension  in  passing  from  one  black  film  to  the  other  has  been 
observed  by  Professors  Remold  and  Riicker.  Based  on 
what  may  be  termed  Lord  Kelvin's*  minimum  surface- 
tension  principle,  they  suppose  the  tension  to  remain  constant 
as  the  films  thin  to  the  limiting  thickness  of  the  coloured 
film.  Then  the  tension  is  supposed  to  fall  as  the  film  continues 
to  thin  until  a  minimum  tension  is  reached,  after  which  the 
tension  rises.  Now  it  is  evident  that,  for  these  thinner  films, 
equilibrium  could  not  be  reached  unless  the  thinning  continues 
so  far  that  the  tension  again  rises  to  that  of  the  coloured 
film. 
On  this  theory  it  should  be  possible  to  have  the  black  film 
in  equilibrium  with  the  coloured  film  of  various  thicknesses. 
Their  experimental  observations  substantiate  this  theory  in 
a  very  interesting  maimer.  If  a  discontinuity  occurs  in  the 
thickness  between  the  successive  black  films,  it  is  reasonable 
to  suppose,  as  Poynting  and  Thomson!  have  remarked,  that 
a  variation  in  the  tension,  similar  to  that  which  has  been 
described,  takes  place. 
Laplace's  theory  of  capillarity  shows  that  the  inference 
to  be  dravvn  is  that  we  have  alternate  molecular  forces  of 
attraction  and  repulsion  as  we  pass  from  one  black  film  to 
the  next.  Or  according  to  the  minimum  surface-tension 
principle,  there  should  be  as  many  undulations  in  the 
curve  representing  thickness  and  tension  as  there  are  black 
films. 
Another  theory  of  these  discontinuities  might  be  proposed, 
which  may  be  little  more  than  an  analogy. 
Long  ago,  Professor  James  Thomson  J,  in  discussing  the 
unstable   condition    supposed  to    exist    between   the    super- 
*  Phil.  Trans.  1886,  Part  ii.  p.  679. 
t  ■  Properties  of  Matter,'  p.  166. 
X  Maxwell.  '  Theory  of  Heat/  p.  127. 
